The =shaft= or =body= (Corpus humeri) is irregularly cylindrical and has a twisted appearance. It may be regarded as having four surfaces. The =external surface= is smooth and is spirally curved, forming the =musculo-spiral groove= (Sulcus musculi brachialis), which contains the brachialis muscle; the groove is continuous with the posterior surface above and winds around toward the front below. The =internal surface= is nearly straight in its length, rounded from side to side, and blends with the anterior and posterior surfaces. Just above its middle is the =internal= or =teres tubercle= (Tuberositas teres), to which the tendon of the latissimus dorsi and teres major muscles is attached. The =nutrient foramen= is in the lower third of this surface. The =anterior surface= is triangular, wide and smooth above, narrow and roughened below. It is separated from the external surface by a distinct border, which bears on its upper part the =deltoid tuberosity= (Tuberositas deltoidea). From the latter a rough line curves upward and backward to the outer surface of the neck, and gives origin to the external head of the triceps muscle. Below the tuberosity the border inclines forward, becomes less salient, and ends at the coronoid fossa. The =posterior surface= is rounded from side to side and smooth. [Illustration: FIG. 45.—LEFT HUMERUS OF HORSE, INTERNAL SURFACE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ] [Illustration: FIG. 46.—LEFT HUMERUS OF HORSE, EXTERNAL SURFACE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]